Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

179
FXUS63 KLOT 280218
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
918 PM CDT Sat Jun 27 2026

.KEY MESSAGES...

- Patchy fog possible (20% chance) tonight, mainly south of the
  Kankakee River Valley.

- Dangerous heat is expected across the area next week.

- Afternoon heat index values forecasted over 100 degrees each
  day next week, with little to no relief at the lakeshore and
  at night.

&&

.UPDATE...
Issued at 918 PM CDT Sat Jun 27 2026

A baggy area of lower pressure continues to slowly meander
across central IL and west-central IN this evening with most of
northern IL and northwest IN residing under the influence of the
broad surface high over the northern Great Lakes. While this
high will make for generally tranquil weather conditions
tonight, a couple of isolated showers may brush portions of
Ford, Iroquois, and Benton Counties through 2 AM CDT or so as
the aforementioned low and its associated warm front drift by.
Otherwise, expect partly to mostly cloudy skies to persist
through the night which will keep temperatures on the mild side
in the mid to upper 60s (warmest south of I-80). Despite the
cloud cover, the area of higher moisture near the warm front may
also support some patchy fog development after midnight into
early Sunday morning. Since confidence in low-level saturation
being reached remains on the low side, especially with northward
extent, have opted to just introduce some 20% chances for fog
south of the Kankakee River for now.

Heading into Sunday, any fog that develops overnight will erode
by 8 AM CDT Sunday morning leaving us with partly cloud skies
for the rest of the day. While mostly dry conditions are
currently being forecast for Sunday, there remains a non-zero
(~10%) chance for the cluster of storms in the Dakotas to drift
into portions of northwest and northern IL Sunday afternoon.
Recent 00z CAM guidance continues to vary on whether or not the
cluster will get far enough east to remain in WI before the
steering flow turns northwesterly (pushing storms towards the
southeast) early Sunday afternoon which continues to make this
part of the forecast low confidence. If the storm cluster does
not dissipate and is able to take advantage of the northwest
flow, the environment over northern IL will be supportive of
showers and thunderstorms with steep mid-level lapse rates
(around 7-8 C/km) and decent moisture courtesy of lower 70s
dewpoints. The caveat however, will be that the warming mid-
levels could provide a cap to surface based thunderstorms which
in turn could limit the coverage and southward progress of the
storm cluster especially if it arrives in a decaying state.
Though, if the cluster is stronger the 30-40 kts of shear and
aforementioned lapse rates could result in a period of strong to
severe storms for portions of the area.

At this time, we still think the more likely scenario for Sunday
is that the storm cluster either stays in WI or dissipates prior
to reaching northern IL keeping dry conditions locally.
Therefore, our current forecast remains dry with just some
"silent" (10-14%) POPs across our northern counties to tease the
shower/storm potential. Obviously this is something we will
continue to monitor overnight, but anyone with outdoor plans
(especially those west of the Fox Valley) should keep a close
eye on the forecast just in case this changes.

Regardless of what happens with the storms, Sunday will be a
warm and muggy day with highs in the mid to upper 80s inland
with mid-70s near the lake. Winds will also be breezy out of the
east-southeast with gusts around 20 mph.

All updated forecast products will be published shortly.

Yack

&&

.DISCUSSION...
Issued at 114 PM CDT Sat Jun 27 2026

A weak area of low pressure, loosely analyzed in south central
Illinois, is taking its sweet time moving east. A pseudo warm
front along and just south of the southern portions of the
forecast area is the main focus of showery precipitation this
morning and afternoon. While the lingering moisture north of
this area will continue to feature a wide swatch of cumulus
clouds, most of the shower activity should remain confined to
the US 24 corridor, and diminish as sunset approaches.

Recent forecasts have been monitoring an incoming EML plume out
of the southern Plains tonight along an axis created by an
amplifying ridge/trough. Models are consistently trending more
hesitant with its northward progression due to lack of moisture
and forcing. While the forecast was kept dry through the night,
closer attention is being paid to what may transpire in South
Dakota tonight. That previously mentioned EML plume will drive a
fairly stout northwest to southeast oriented MUCAPE axis into
Sunday. There has been a notable uptick in model activity
developing a MCS in South Dakota tonight and have it moving
eastward. While it is a small percentage of models, some have it
riding the instability gradient southeastward into northwest
Illinois. For now, confidence is low in the threat for showers
or storms tomorrow afternoon. It would take a fairly strong MCS
to hold itself together and fight the drier air mass expected
over Illinois tomorrow, and even the models that show it
entering Illinois have it arriving in a decaying state. But with
plenty of instability and shear, PoPs were increased to at
least have "silent" 10 to 14 percent chances in the northern
half of the forecast area.

The main focus of the forecast will continue to be the first
heat wave of the year. The aforementioned amplifying upper level
pattern will continue today and tomorrow and feature a strong
upper level low to the west, and an upper level high embedded in
a ridge to the east. This pattern will advect around 25C
temperatures at 850 mb into the area and turn the prevailing
wind pattern to the southwest. This warmer, drier air mass is
expected to remain for much of next week. Widespread
temperatures in the 90s, and dew points well into the 70s will
create hot and humid conditions. This will create afternoon heat
index values around 105F each day starting on Monday. Little to
no relief can be expected at the lakeshore with the offshore
flow, and nighttime will not help either as overnight lows are
only expected in the mid to upper 70s. It is becoming
increasingly likely that a Heat Advisory will be needed across
the forecast area, but for the time being, an Excessive Heat
Watch was issued for Cook County from Monday afternoon through
Wednesday evening.

So, if confidence in an Advisory is increasing, why was one not
issued? Why did Cook County get a Watch? Is a Warning possible
for the rest of the forecast area? First, Cook County has a
different criteria for Heat watches and warnings, given the
population size. It only takes heat indices of 105F for
only two days or three days of 100F to justify a watch/warning.
The rest of the forecast area needs heat indices of 110F for a
watch/warning, and unless dew point temperatures over-perform,
that appears unlikely. Lastly, there is uncertainty with the MCS
tomorrow on its progression and if it lingers into Monday.
Additionally, models are trending farther east with the upper
level ridge and having it start to break down somewhere in the
middle of the week. This further adds questions on the duration
of the event, as a weaker and/or more eastern ridge would open
the door to additional convection that could provide lower heat
indices in the 90s and up to 100 (which is still hot, just sub-
criteria). Meanwhile, a stronger ridge could make for a longer
duration heat event that could last until the holiday weekend
next week.

So the main message: the first heat wave of the year is
expected next week. Take the hot temperatures into account when
planning activities through the week.

DK

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 633 PM CDT Sat Jun 27 2026

Forecast concerns for the terminals...

- Chance for patchy 5-6 SM fog at GYY with better fog coverage
  likely south of the Kankakee River Valley.

- Breezy east-southeast winds Sunday afternoon with 17-20 kt
  gusts.


Generally quiet weather is expected for the TAF period as high
pressure will begin to build into the area on Sunday. In the
meantime, some lingering mid-level moisture will maintain some
4000-5000 ft BKN to OVC skies through tonight before clouds
scatter out Sunday afternoon. While there is a chance for some
MVFR clouds to develop and lift out of central IL tonight,
confidence on their coverage with northward extent remains low.
Thus have opted to tease this potential with a SCT030 mention at
in the TAFs. In addition to the clouds, there remains a chance
for some patchy fog to develop tonight as the humid air mass to
our south progresses northward. With the expected cloud cover
limiting radiational cooling suspect that the better fog
coverage will remain south of the Kankakee River Valley tonight.
However, have opted to maintain a 6 SM BR mention at GYY where
cooler air off the lake may aid in some patchy fog development
late tonight into early Sunday morning.

Otherwise, expect light east-northeast winds tonight to increase
and become east-southeast on Sunday. A few 17-20 kt gusts are
also expected for Sunday afternoon before winds taper to around
7-9 kts Sunday evening.

Lastly, any earlier concerns for some spotty showers to develop
along the elevated instability axis late tonight into early
Sunday morning continue to wane. Therefore, we expect dry
conditions to prevail through Sunday. However, there is a new
concern for the cluster of storms that is developing in the
Dakotas to drift into portions of northwest and northern IL
Sunday afternoon if it holds together. While the steering flow
should take this cluster into central WI, there is a play that
if the cluster is still in southern MN Sunday morning that the
veering steering flow may allow some of these showers/storms to
drift into northwest and northern IL. Right now confidence is
very low (<10% chance) that any showers/storms even make it into
northwest IL, but this will be something to keep a lazy eye on
just in case.

Yack

&&

.CLIMATE...
Issued at 252 AM CDT Sat Jun 27 2026

     .Daily Records for the Upcoming Week........

Chicago (KORD)
            Record High       Record Warm Minimum
June 29       97 (1954)            80 (1931)
June 30       99 (1953)            80 (2018)
July 1       103 (1956)            78 (1931)
July 2        99 (1970)            79 (1911)
July 3       102 (1949)            78 (1911)
July 4       102 (2012)            80 (1921)


Rockford (KRFD)
            Record High       Record Warm Minimum
June 29      103 (1931)            75 (1931)
June 30      102 (1931)            76 (2018)
July 1       101 (1931)            76 (1970)
July 2        99 (1910)            74 (1970)
July 3        99 (1925)            75 (2012)
July 4       102 (2012)            77 (1977)

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Extreme Heat Watch from Monday afternoon through Wednesday
     evening for ILZ103-ILZ104-ILZ105.

IN...None.
LM...None.

&&

$$

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